Headsets have long been used in conjunction with modem telephone equipment. Typically, headset users are ones whose jobs require either that they spend a substantial amount of time on the phone, or that their hands be free to perform other tasks. Examples of occupations where headsets are commonly used, include, among others, telephone operators, office receptionists, airline reservation clerks, stock brokers, customer service representatives, and police and fire department dispatchers.
The use of a headset provides many advantages. For example, a headset allows the user to perform other tasks while on the telephone. Examples of such tasks include taking messages, routing incoming calls, and using a computer terminal to enter or retrieve data. The use of a headset also reduces the amount of time required to answer an incoming call, thus making the headset user more efficient. In addition, the use of a headset eliminates the physical discomfort that can occur when a person talking on the telephone attempts to prop a regular telephone handset against his or her ear by tilting the head and raising the shoulder.
However, there are also disadvantages associated with the use of headsets. Since the headset's transducer (i.e. a speaker) is held in or against the user's ear, it is impossible to respond to any irritating tones or noises by moving the transducer closer to, or further away from the ear as one typically does with a regular telephone handset.
For example, if a calling party activates the dual tone multi-frequency (DTMF) (tone dialing) keypad while his or her telephone station is connected to an operator wearing a headset, a tone signal as loud as--3 dBm to 0 dBrn can be generated on the telephone line at the operator's station. Often, aside from being annoyingly loud, such a tone is of a frequency that startles and annoys the headset wearer.
It is well known that excessive exposure to high-frequency noise causes fatigue and difficulty in concentration on one's work. This is particularly true if one experiences bursts of loud high-frequency noises, because human ears are most sensitive to frequencies in the range of 1-6 KHz. For voice grade telecommunications, high-frequency signals are defined to be in the range of 2-4 KHz. Thus, a reduction in exposure to high-frequency tones is desirable for both the comfort of employees wearing telephonic headsets and to prevent such employees from being unnecessarily fatigued and to meet the requirements of the Occupational Safety and Health Administration (OSHA).
There are existing headset control circuits which address the problem of loud sounds. One is a peak limiting circuit which suppresses extremely loud signals. A peak limiting device redtrees the level of signals which exceed a predetermined level. Only those signals (voice or noise) which are extremely loud are attenuated by a peak limiter. This type of device prevents the headset user from suffering discomfort or injury which could be caused by excessively loud signals. However, this approach does not attenuate the lower level but annoying and sometimes startling high-frequency signals. These circuits fail to attenuate high-frequency signals and pass lower frequency signals, the goal desired. Additionally, most limiting circuits are clipping devices and simply clip off the excursion of a signal past a particular threshold causing odd harmonic distortion which is known to have a harsh sound to the listener.
Conventional automatic gain control (AGC) works in a well known manner to make the gain at a given stage of amplification a function which is inversely proportional to the signal level at a given point in the circuit. Most conventional AGC circuits simply feed back the output of a particular stage to provide a control signal which reduces the gain as the output increases. In most application of AGC circuits to audio signal paths, the AGC is simply a form of negative feedback and is most commonly used to provide a non-linear (normally approximating logarithmic) signal level to gain characteristic.
As is well known to those skilled in the art, automatic gain control circuits are devices which almost always respond to some form of integrated or average signal level. Those which respond very quickly may be thought of as devices having very short integration times, and thus are circuits which average a very short time window of the signal level. Slower responding devices make the gain a function of the history of the signal over a longer, most recent interval, i.e., they have a longer integration time. Those skilled in the art know that it is common to define and describe two characteristics of automatic gain control known as attack and release time. The attack time is the time period required after a sudden increase in the input signal amplitude for the gain of the AGC circuit to reach a predetermined percentage of the steady state change in gain it will make in response to continued application of the new input signal level. Similarly, release time is defined as a similar interval for the change in amplification which results from a sudden decrease in input signal level. Those skilled in the art know that many AGC circuits are designed to achieve varying desired attack and release times without introducing an excessive amount of distortion into the audio signal. Unfortunately, AGC's typically do not have quick attack times, which permits short bursts of high-frequency tones to pass through before responsive attenuation can occur.
Also, the goal of AGC circuits is dynamic range compression. Range compression serves to reduce the amplitude difference in audio signal between the loudest and softest signals present in the signal path. Although AGC's compress audio signals, unexpected high-frequency tones still cause problems for headset wearers. Even lower amplitude high-frequency signals often cause discomfort. In summary, both peak limiting and AGC's circuits act over the whole frequency spectrum, and do not take into account the ears added sensitivity to the higher frequency signals. Thus, a fast attack attenuator which only responds to and reduces higher frequency tones is needed.
Currently, two methods for reducing the effects of high-frequency tones exist. First, a low-pass filter could be placed on input signals. This would attenuate all higher frequency inputs. However, as one would expect, desired voice signals are attenuated and, thus, intelligibility is reduced and the output is distorted. This method is not acceptable.
Second, "Dolby-type" frequency sensitive compressors have been used in the recorded sound arts. (U.S. Pat. No. 3,631,365). The well known Dolby compression and expander system for use in recorded audio provides amplitude sensitive filtering over specified segments of the audio band width. The most common Dolby system is "Dolby B", which is designed for use with home magnetic tape recording equipment. The Dolby compressor/expander system is one for which low level inputs signals have their high frequency content expanded. In other words the gain over the higher frequency portion of a band width of interest is increased in response to a low signal level. This expanded signal is recorded on tape.
On playback, detection of a low intensity signal causes a complimentary compression of the same high frequency band. This has the effect of attenuating the high frequency noise, or "tape hiss," inherent in the recording process, thereby reducing the perceived noise level. Dolby B system does not attenuate at any frequency in the presence of high intensity signals because the high level signal will mask the tape hiss. Thus, an expanded signal is recorded, but only when the input signals is at a relatively low level. This prevents expansion of relatively high level signals in a manner that would cause clipping or saturation during the recording process.
Thus, while Dolby teaches frequency selective attenuation to reduce perceived noise levels, it does so only in the context of a complimentary expander/compressor for use in recording and playing back audio signals.
Therefore, it can be seen that a need yet exists for a high- frequency tone attenuator for headsets, wherein the attenuation of high-frequency tones is relatively fast and minimum attenuation of low-frequency signals occurs. Further, the attenuation should occur only when high-frequency input energy exceeds low-frequency input energy.